The analysis of a sample solution can be accomplished in seVeral ways. Various procedures, to include chemical analysis of a selected sample, are well known in the pertinent art. Chemical analyses, however, generally exhaust the sample and cannot therefore be used since it is often desirable to neither alter nor destroy the tested sample. This is particularly so when the sample agent in solution is very valuable and only small amounts of the sample are available. In such cases it is preferable to perform nondestructive testing.
It is known that, in many instances, sample solutions have individually identifying characteristics which can be observed without destroying or altering the sample. In general, to perform nondestructive testing it is necessary to subject the sample to a known input and observe the change caused by the sample as manifested by the output. It is, of course, important to precisely identify this input. Knowing the input, it is then simply a matter of accounting for components of this input which are missing from a measured output. In the case of sample solutions, various procedures can be used to determine which components of the predetermined input are absorbed by the sample. For example, it is well known that certain sample solutions absorb particular wavelengths of light in varying degrees of intensity and a spectrophotometer can be used to make use of this fact for the analysis of sample solutions. Light absorption, however, is but one way in which the absorption characteristics of a sample solution can be used to analyze the sample. It is also known that microwave power can be used for this purpose. As with a spectroanalysis, a microwave analysis requires comparison of an output to a known input.
Heretofore, both spectroanalysis and microwave analysis techniques and their attendant procedures have required relatively large sample sizes in order to perform an accurate and precise analysis. This, however, can be a problem when the sample is necessarily very small or is consequently very expensive. Unfortunately, this difficulty is more commonplace now than before due to the increased interest for research in many diverse, highly technical fields of science, e.g., biotechnology. Thus, there is a need for a diagnostic device which can accurately analyze an extremely small sample solution.
The present invention recognizes that sample sizes as small as 0.003 microlitres of solution can be accurately analyzed using microwave technology. Specifically, the present invention recognizes the need for such analysis by a diagnostic device which can measure and analyze a sample solution while it is held in the small capillary tubes that are typically used in research laboratories for handling and transferring minute quantities of sample solutions. The present invention accomplishes this by using a device which incorporates a coaxial geometry.
Theoretically, it has been determined for a diagnostic device having a coaxial geometry that the microwave power absorbed per volume of sample solution can be expressed as: ##EQU1## where .omega. is the microwave radial frequency, .epsilon." is the imaginary part of the dielectric constant, P.sub.in is the input microwave power, R is the impedance of the coaxial device, "a" is the outer radius of the inner conductor, and "b" is the inner radius of the outer conductor. Interestingly, this expression neglects reflected power in the device. It happens, however, that with very small sample sizes, and consequently very small sample holders, the reflected power is minimal and is effectively lost within the system noise. Stated differently, with extremely small sample holders, the perturbation to the system which is caused by the sample will be small since proper impedance matching is maintained. Accordingly, for diagnostic purposes, the above expression is acceptably accurate.
In light of the above, it is an object of the present invention to provide a coaxial microwave absorption diagnostic device which is compact and easy to use. Another object of the present invention is to provide a coaxial microwave absorption diagnostic device which is able to measure and analyze extremely small sample solution volumes. Yet another object of the present invention is to provide a coaxial microwave absorption diagnostic device which can analyze a sample solution over a wide range of microwave frequencies. Still another object of the present invention is to provide a coaxial microwave absorption diagnostic device which is reliable and accurate. Another object of the present invention is to provide a coaxial microwave absorption diagnostic device which can be rapidly operated and which is not time consuming. Yet another object of the present invention is to provide a coaxial microwave absorption diagnostic device which is relatively easy to manufacture and which is comparatively cost effective.